1. Field
The present invention relates to a hollow-core photonic crystal fibre (HCPCF) with a large pitch. By “large pitch” it is meant that the photonic structure has a pitch of a size of the order of five times or more as large as the wavelength of light at which the HCPCF operates. The present invention also relates to a HCPCF with a transverse structure comprising relatively thin connecting microcapillary walls. The present invention further relates to methods of manufacturing an HCPCF.
2. Background Art
A currently known HCPCF has a hollow core surrounded by a cladding of silica microcapillaries and creates a photonic band gap (PBG), trapping in the core defect electromagnetic waves within a particular range of optical wavelengths. HCPCFs are theoretically designed around two balancing factors: the need for a large PBG at the wavelength of interest and the feasibility of manufacturing the fibre with the methods available. Out of all known cladding structures, the best tradeoff between these two factors is a triangular lattice HCPCF.
An example of such a triangular lattice HCPCF has a triangular arrangement of air holes in a silica background, an air-filling fraction exceeding 90% and a pitch (Λ), which is the distance between the centres of the air holes in the cladding structure, approximately twice as large as the wavelength of operation (λ). For example, for one such HCPCF a pitch of 3-4 μm is required for a maximum of ˜300 nm wide band gap centred at an infra-red (IR) wavelength of 1550 nm.
The guidance properties of such HCPCFs have resulted in applications such as efficient Raman lasers, gas-laser fibre devices, high power soliton delivery and efficient electromagnetically induced transparency.
However the pitch required to achieve fibre guidance in the visible or ultra-violet (UV) region of the optical spectrum is 2 to 4 times smaller than that of a fibre guiding in the IR. This is because the PBG is located at a relatively low normalized frequency kΛ˜12, where k, the propagation constant, =2π/λ, with the pitch scaling linearly with the guided wavelength. In general the pitch must be less than 2 μm for any known fibre guiding in the visible or UV.
The fabrication of such HCPCFs poses several engineering challenges. Firstly, the air filling fraction of the cladding structure required to maximize the PBG's optical wavelength span needs to be >90%. This often involves etching of capillaries or the insertion of glass rods through the interstitial holes between the thin capillaries during stacking to create the required structure. Secondly, the core mode couples with interface modes located at the core surround, which affects the guidance dramatically. This means that the core shape, thickness and size need to be accurately designed and controlled during fabrication. Thirdly, the small pitch required is difficult to control accurately during the fibre drawing process.
Because of the manufacturing techniques used (conventionally, capillaries are blown and then etched with a gas such as gaseous hydrogen fluoride (HF)), these small pitch triangular lattice HCPCFs also suffer from the problem that their optical properties are not optimum, for example because undesired modes may be present. These issues inherent to fabricating visible and UV guiding HCPCFs and the overlapping of guided light with silica limit their loss (optical attenuation) to about 1 dB/m.
As well as the fabrication difficulties, there are also other drawbacks that prevent such triangular lattice fibres from being used in applications requiring broadband guidance or guidance in the visible or UV. These include the intrinsically narrow optical transmission bandwidth of the fibre, the overlap between the fundamental mode and interface modes and the high light-in-silica fraction.